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Converts web_state-related files to ARC.

Notable issues:
 - Had to fix WebUIMojoTest.MessageExchange. See comment.

Bug: 
Change-Id: Ia8a58433ab82d67ebae4f61b8c4ea954f84df202
Reviewed-on: https://chromium-review.googlesource.com/570402
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487544}